Sensei

Sensei is the human who created the Servamps. He was killed by Sleepy Ash hundreds of years ago before the main story.

Appearance
While his appearance in manga hasn't been shown, in the anime, he closely resembles Kuro, having short bluish white hair, wearing white shirt, grey pants, a pair of black boots, and white haori over his body.

Background
Sensei was a researcher born hundreds of years ago. So far, his only known research is about vampires. This research resulted him capable of changing seven humans into a special vampires known as Servamps that each represents the seven deadly sins. However, not all Servamps were happy with their change, particularly Sleepy Ash of Sloth who despite having a close relationship with Sensei, views the existence of monsters such as vampires are unnecessary, and All of Love of Lust who view his change as a punishment since he doesn't wish to live anymore yet was turned into an immortal being.

Despite being a human, he was able to live for a hundred years without aging. During this time, he continued his research in Japan and then changed another human into a Servamp named Tsubaki who represents Melancholy. Since the other Servamps doesn't know of their existence prior their meeting with Tsubaki, it's presumed that Sensei never told any of them about their youngest sibling and kept his existence hidden from them. Furthermore, unlike the other Servamps, Tsubaki was created with the ability to break the contract item that constitutes the link between Servamps and Eves. One day, a neutral organization, C3, sent a request to the seven Servamps, which was to kill their creator. The opinions between the Servamps were divided, so they decided to do a majority vote. The Mother of Wrath, Lawless of Greed, and World End of Gluttony objected due to their attachment to their creator, while Old Child of Pride, Doubt Doubt of Envy, and All of Love agreed out of fear that their creator may create more monsters like them in the future. Sleepy Ash, who held the final vote, also thinks that their creator's research is too dangerous and has to be stopped, so he decided to personally kill their creator. Arriving in Japan, Sleepy Ash confronted Sensei in his house, and Sensei greeted him warmly. Despite his uncertainty, Sleepy Ash still sticked with his decision and killed his creator immediately after he arrived.

Sensei's death left a great impact on Sleepy Ash who soon regretted his action afterwards, and Tsubaki who since then determined to fulfil Sensei's expectation to break the other Servamps and fix their messed up world, though whether Sensei really requested Tsubaki to do so or not is not yet confirmed.